In the conventional hot rolling line, the sheet bar to be rolled is heated to a target temperature and then fed into rough rolling step and a finish rolling step every the bar to obtain a hot rolled sheet having a desired thickness. In such a rolling system, it is apt to cause troubles such as line stopping due to poor biting of a rolling material in the finish rolling and the like, and also the lowering of yield resulted from poor shapes of front and back end portions of the rolling material is large. Therefore, it is desired to prematurely solve these problems.
As an attempt for solving the above troubles and more improving the productivity in the hot rolling line, there is proposed a rolling technique as disclosed in Japanese Patent laid open No. 60-244401 or Japanese Patent laid open No. 61-159285.
In the technique disclosed in the above publications, the back end of the preceding rolling material and the front end of the succeeding rolling material are joined at the entrance side of the finish rolling step, whereby the several to several ten rolling materials are continuously fed into the finish rolling step. According to such a rolling manner, there is caused no poor biting as generated in case of feeding the sheet bars into the rolling line every the bar and the productivity is largely improved.
However, when a high frequency heating is carried out at portions of the sheet bars to be joined as disclosed in Japanese Patent laid open No. 60-244401, the heating up to a target joining temperature can be attained in a relatively short time, but an extra region other than the region to be joined in the sheet bar is also heated, so that an energy consumption amount used therefor becomes too large. On the otherhand, when electric heating is used as disclosed in Japanese Patent laid open No. 61-159285, if a scale is adhered to the surface of the sheet bar, arc may be generated by contact electric resistance between the sheet bar and electrode roll to damage the electrode roll. In any case, these techniques are insufficient for realizing the continuous hot rolling of the sheet bar.
As to the joining apparatus of the sheet bars, the formation of large-size apparatus is not avoided in the conventional technique, so that it is difficult to apply such an apparatus to the existing line, and also the structure itself becomes complicated, which is poor in its maintenance.
It is an object of the invention to provide a novel joining method capable of simply, rapidly and surely joining sheet bars without wastefully consuming energy and without causing breakage of joint portion between sheet bars in rolling as well as an apparatus directly used for carrying out this method.